The present invention provides a grain-oriented electrical steel sheet containing from 2.0 to 7.0% of Si and excellent in film characteristics and iron loss characteristics. Moreover, the present invention provides a process for producing a grain-oriented electrical steel sheet extremely excellent in film characteristics and excellent in iron loss characteristics by controlling the initial oxide film of a steel strip which has been rapidly heated in the heating stage for decarburization annealing prior to introducing the steel strip into the decarburization annealing furnace. Furthermore, the present invention provides a decarburization annealing facility used in the production process. The present invention relates to the products, the production process and the facility.
The magnetic characteristics of grain-oriented electrical steel sheets are generally evaluated for both iron loss and excitation characteristics. Improving the excitation characteristics is effective in downsizing an apparatus of which the designed magnetic flux density is to be increased. On the other hand, decreasing the iron loss is effective in reducing the energy lost as thermal energy and saving power consumption during the use of the steel sheet in electrical appliances. Moreover, aligning the  less than 100 greater than  orientation of the grains of the product improves the excitation characteristics and lowers the iron loss. Many investigations have been carried out in this field in recent years, and various products and production technologies have been developed.
For example, Kokoku (Japanese Examined Patent Publication) No. 40-15644 discloses a process for producing a grain-oriented electrical steel sheet for obtaining a high magnetic flux density. In the process, AlN+MnS functions as an inhibitor, and the steel sheet is forcibly rolled with a reduction ratio exceeding 80% in the final cold rolling step. According to the process, the density of the {110} less than 001 greater than  orientation of the secondary recrystallization is high, and a grain-oriented electrical steel sheet having a high magnetic flux density of at least 1.870 T in terms of B8 can be obtained.
However, although the iron loss can be decreased to some extent by the production process, the macroscopic grain diameter of secondary recrystallized grains is of the order as large as 10 mm. As a result, the eddy-current loss which is a factor influencing the iron loss cannot be decreased, and a superior iron loss has not been obtained.
In contrast to the process mentioned above, Kokoku (Japanese Examined Patent Publication) No. 6-51187 discloses a process for making secondary recrystallized grains smaller to improve the magnetic characteristics. The process comprises ultrarapidly annealing a steel sheet (strip) which has been rolled at an ambient temperature at temperatures of at least 657xc2x0 C. at a heating rate of at least 140xc2x0 C./sec, decarburizing the steel sheet, and final annealing the steel sheet at high temperatures so that secondary grain growth takes place, whereby the steel sheet contains secondary grains having a decreased size and has a lasting improved iron loss without a significant change even after stress relieving annealing.
However, it is difficult to obtain an electrical steel sheet exhibiting an iron loss comparable to that of an electromagnetic steel sheet having fine magnetic domains, by merely converting the secondary grains into fine ones by the production process. In particular, in final annealing where the steel sheet is rapidly exposed to high temperatures by rapid heating to form an oxide film having a different composition and to preferentially form fayalite (Fe2SiO4), coating the steel sheet with MgO does not necessarily result in an excellent formation of forsterite (2MgOxe2x80xa2SiO2). As a result, there arises the problem that excellent magnetic characteristics cannot be obtained due to an insufficient film tension.
In order to solve such a problem, Kokai (Japanese Unexamined Patent Publication) No. 7-62436 proposes the following method: directly before annealing a steel strip having been rolled to a final sheet thickness or in a heating stage of decarburization annealing, the steel strip is heated to at least 700xc2x0 C. at a heating rate of at least 100xc2x0 C./sec in a nonoxidizing atmosphere having a PH2O/PH2 ratio of up to 0.2, and heat treated. Moreover, the patent publication also proposes the use of two pairs of conductor rolls as a concrete example of rapid heating.
However, it has been found that a dense oxide layer is sometimes formed on the steel sheet during rapid heating in such a production method. When such an oxide layer is formed, it becomes a barrier, and influences the decarburization. In particular, decarburization of a magnetic steel sheet having a residual C content of up to 40 ppm becomes difficult. As a result, the magnetic characteristics of the products are deteriorated due to magnetic aging, although an electrical steel sheet having excellent magnetic characteristics can be obtained immediately after the production. Moreover, it becomes impossible to sufficiently decarburize the steel sheet to have a residual C content of up to 20 ppm even by extending the decarburization time.
Furthermore, a grain-oriented electrical steel sheet is generally bent when wound cores are prepared therefrom and incorporated into transformers, etc. Accordingly, the electrical steel sheet is required to have such an excellent film adhesion, particularly at the corner portions having a large curvature, that no peeling of the surface film consisting of a primary film and a secondary film (insulating coating) takes place. In the production process mentioned above, there is still room for improving the film adhesion.
The present invention provides a grain-oriented electrical steel sheet containing from 2.0 to 7.0% of Si and excellent in film characteristics (film adhesion) and magnetic characteristics (iron loss characteristics), a process for producing the same, and a decarburization annealing facility used for the production process.
In order to obtain a grain-oriented electrical steel sheet excellent in both the film characteristics (film adhesion) and the magnetic characteristics (iron loss characteristics), the present inventors carried out many tests wherein a steel strip rolled to have a final product thickness was rapidly heated to at least 800xc2x0 C. at a heating rate of at least 100xc2x0 C./sec in the heating stage in the decarburization step.
The tests were carried out using a decarburization annealing facility prepared by altering a conventional decarburization annealing furnace which had already been installed and was generally used for practicing a decarburization annealing step and which had, on the steel strip entry side (usually within 5 m from the steel strip inlet), an exhaust vent to the atmosphere.
That is, the tests were carried out using a decarburization annealing facility, wherein a rapid heating chamber provided with an apparatus for conducting the rapid heating was connectively provided to the entry side of a decarburization annealing furnace having already been installed with or without a throat portion provided between the furnace and the chamber, and the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace were exhausted through the exhaust vent mentioned above.
During conducting the decarburization annealing step using the decarburization annealing facility, investigations were made on the relationships between an atmosphere of the rapid heating chamber (including the throat portion when provided), an atmosphere of the decarburization annealing furnace, a residence time of the steel strip at temperatures of at least 750xc2x0 C. in the rapid heating chamber (including the throat portion when provided), a film adhesion of the product and iron loss characteristics prior to and subsequent to magnetic aging. As a result, the following discoveries have been made.
1) A product excellent in characteristics shows that the peak position of Si from the oxide film surface is up to {fraction (1/10)} of the peak position of Al therefrom on the surface layer side when subjected to glow discharge spectral analysis (GDS analysis).
2) A product still more excellent in characteristics shows that the peak position of Si from the oxide film surface is up to {fraction (1/20)} of the peak position of Al therefrom on the surface layer side when subjected to glow discharge spectral analysis (GDS analysis).
3) An oxide film satisfying the characteristics in 1) can be obtained by the following procedure: an annealing facility is used in which the decarburization annealing furnace is provided, near the entry side thereof, with an exhaust vent for exhausting the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace; the PH2O/PH2 ratio is held at 0.20 to 3.0 in the rapid heating chamber; the PH2O/PH2 ratio is held at 0.25 to 0.6 in the decarburization annealing furnace; and the residence time of the steel strip at temperatures of at least 750xc2x0 C. is held within 5 sec in the rapid heating chamber.
4) An oxide film satisfying the characteristics in 2) can be obtained by the following procedure: an annealing facility is used in which the decarburization annealing furnace is provided, near the entry side thereof, with an exhaust vent for exhausting the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace; the PH2O/PH2 ratio is held at 0.8 to 1.8 in the rapid heating chamber; the PH2O/PH2 ratio is held at 0.25 to 0.6 in the decarburization annealing furnace; and the residence time of the steel strip at temperatures of at least 750xc2x0 C. is held within 5 sec in the rapid heating chamber.
The present invention is based on the discoveries, and the features of the invention are as described below.
(1) A grain-oriented electrical steel sheet which has excellent film characteristics and magnetic characteristics,
comprising up to 0.005% of C, 2.0 to 7.0% of Si in terms of weight % and the balance Fe and unavoidable impurities,
having an oxide film which mainly contains forsterite and is formed on the surface, and an insulating coating formed on the oxide film,
wherein the amount of the oxide film is from 1 to 4 g/m2 per side, and the depth of the peak position of Si from the oxide film surface is up to {fraction (1/10)} of the depth of that of Al therefrom in glow discharge spectral analysis (GDS analysis) from the oxide film surface, and
showing a ratio y (%) with which peeling of the oxide film does not take place when subjected to a bending test with a curvature of 20 mm and which satisfies the following formula (1):
y(%)xe2x89xa7xe2x88x92122.45t+112.55xe2x80x83xe2x80x83(1) 
wherein t represents a sheet thickness in terms of mm, and iron loss characteristics W (W/kg) which satisfy the following formula (2):
W (W/kg)xe2x89xa62.37t+0.280xe2x80x83xe2x80x83(2) 
wherein t represents a sheet thickness in terms of mm.
(2) The grain-oriented electrical steel sheet which has excellent film characteristics and magnetic characteristics as disclosed in (1), wherein the depth of the peak position of Si from the oxide film surface is up to {fraction (1/20)} of the depth of that of Al therefrom, and the electrical steel sheet shows a ratio y (%) with which peeling of the oxide film does not take place when subjected to a bending test with a curvature of 20 mm and which satisfies the following formula (3):
y (%)xe2x89xa7xe2x88x92122.45t+112.55xe2x80x83xe2x80x83(3) 
wherein t represents a sheet thickness in terms of mm, and iron loss characteristics W (W/kg) which satisfy the following formula (4):
W (W/kg)xe2x89xa62.37t+0.260xe2x80x83xe2x80x83(4) 
wherein t represents a sheet thickness in terms of mm.
(3) In a process for producing a grain-oriented electrical steel sheet comprising the step of conventionally treating a slab comprising up to 0.10% of C, 2.0 to 7.0% of Si in terms of weight %, up to 400 ppm of Al, a conventional inhibitor component, and the balance Fe and unavoidable impurities and rolling to form a steel strip having a final product thickness, the step of decarburization annealing the steel strip, the step of final finishing annealing the steel strip and the step of conducting an insulating coating treatment, a process for producing a grain-oriented electrical steel sheet which has excellent film characteristics and magnetic characteristics as disclosed in (1), characterized in that:
the steel strip is rapidly heated to temperatures of at least 800xc2x0 C. at a rate of at least 100xc2x0 C./sec by subjecting the steel strip to a heating stage in the decarburization annealing step in a rapid heating chamber which is connectively provided to a decarburization annealing furnace while the PH2O/PH2 ratio is held at 0.20 to 3.0 and the residence time of the steel strip at temperatures of at least 750xc2x0 C. is set within 10 sec in the rapid heating chamber; and
the steel strip is decarburization annealed in a decarburization annealing furnace provided with an exhaust vent near the entry side which exhausts the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace, while the PH2O/PH2 ratio is held at 0.25 to 0.6 in the decarburization annealing furnace.
(4) In a process for producing a grain-oriented electrical steel sheet comprising the step of conventionally treating a slab comprising up to 0.10% of C, 2.0 to 7.0% of Si in terms of weight %, up to 400 ppm of Al, a conventional inhibitor component, and the balance Fe and unavoidable impurities and rolling to form a steel strip having a final product thickness, the step of decarburization annealing the steel strip, the step of final finish annealing the steel strip and the step of conducting an insulating coating treatment, a process for producing a grain-oriented electrical steel sheet which has excellent film characteristics and magnetic characteristics as disclosed in (2), characterized in that:
the steel strip is rapidly heated to temperatures of at least 800xc2x0 C. at a rate of at least 100xc2x0 C./sec by subjecting the steel strip to a heating stage in the decarburization annealing step in a rapid heating chamber which is connectively provided to a decarburization annealing furnace while the PH2O/PH2 ratio is held at 0.8 to 1.8 and the residence time of the steel strip at temperatures of at least 750xc2x0 C. is set within 5 sec in the rapid heating chamber; and
the steel strip is decarburization annealed in a decarburization annealing furnace provided with an exhaust vent near the entry side which exhausts the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace, while the PH2O/PH2 ratio is held at 0.25 to 0.6 in the decarburization annealing furnace.
(5) In a process for producing a grain-oriented electrical steel sheet comprising the step of conventionally treating a slab comprising up to 0.10% of C, 2.0 to 7.0% of Si in terms of weight %, up to 400 ppm of Al, a conventional inhibitor component, and the balance Fe and unavoidable impurities and rolling to form a steel strip having a final product thickness, the step of decarburization annealing the steel strip, the step of final finishing annealing the steel strip and the step of conducting an insulating coating treatment, a process for producing a grain-oriented magnetic steel sheet which has excellent film characteristics and magnetic characteristics as disclosed in (1), characterized by that:
the steel strip is rapidly heated to temperatures of at least 800xc2x0 C. at a rate of at least 100xc2x0 C./sec by subjecting the steel strip to a heating stage in the decarburization annealing step in a rapid heating chamber which is connectively provided to a decarburization annealing furnace through a throat portion, while the PH2O/PH2 ratio is held at 0.20 to 3.0 and the residence time of the steel strip at temperatures of at least 750xc2x0 C. is set within 10 sec in the rapid heating chamber and throat portion; and
the steel strip is decarburization annealed in a decarburization annealing furnace provided with an exhaust vent near the entry side which exhausts the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace, while the PH2O/PH2 ratio is held at 0.25 to 0.6 in the decarburization annealing furnace.
(6) In a process for producing a grain-oriented electrical steel sheet comprising the step of conventionally treating a slab comprising up to 0.10% of C, 2.0 to 7.0% of Si in terms of weight %, up to 400 ppm of Al, a conventional inhibitor component, and the balance Fe and unavoidable impurities and rolling to form a steel strip having a final product thickness, the step of decarburization annealing the steel strip, the step of final finish annealing the steel strip and the step of conducting an insulating film treatment, a process for producing a grain-oriented electrical steel sheet having excellent film characteristics and magnetic characteristics as disclosed in (2), characterized in that:
the steel strip is rapidly heated to temperatures of at least 800xc2x0 C. at a rate of at least 100xc2x0 C./sec by subjecting the steel strip to a heating stage in the decarburization annealing step in a rapid heating chamber which is connectively provided to a decarburization annealing furnace through a throat portion, the PH2O/PH2 ratio in the rapid heating chamber and the throat portion being held at 0.8 to 1.8, while the residence time of the steel strip at temperatures of at least 750xc2x0 C. is set within 10 sec in the rapid heating chamber and throat portion; and
the steel strip is decarburization annealed in a decarburization annealing furnace provided with an exhaust vent near the entry side which exhausts the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace, while the PH2O/PH2 ratio is held at 0.25 to 0.6 in the decarburization furnace.
(7) The process for producing a grain-oriented electrical steel sheet having excellent film characteristics and magnetic characteristics as disclosed in (3) to (6), wherein the rapid heating is carried out by conducting heating through directly applying a current using conductor rolls.
(8) The process for producing a grain-oriented electrical steel sheet having excellent film characteristics and magnetic characteristics as disclosed in (3) to (7), wherein magnetic domain refinement treatment is conducted.
(9) A decarburization annealing system for a grain-oriented electrical steel sheet comprising a rapid heating chamber internally provided with a rapid heating apparatus which heats a steel strip having been rolled to have a final product thickness to temperatures of at least 800xc2x0 C. at a rate of at least 100xc2x0 C./sec, and a decarburization annealing furnace for conducting decarburization annealing which is connectively provided to the rapid heating chamber and which has, near the entry side of the furnace, an exhaust vent for exhausting the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace.
(10) A decarburization annealing facility for a grain-oriented electrical steel sheet comprising a rapid heating chamber internally provided with a rapid heating apparatus which heats a steel strip having been rolled to have a final product thickness to temperatures of at least 800xc2x0 C. at a rate of at least 100xc2x0 C./sec, and a decarburization annealing furnace for conducting decarburization annealing which is connectively provided to the rapid heating chamber through a throat portion and which has, near the entry side of the furnace, an exhaust vent for exhausting the atmosphere of the rapid heating chamber and that of the decarburization annealing furnace.
(11) The decarburization annealing facility for a grain-oriented electrical steel sheet as disclosed in (9) or (10), wherein the apparatus for conducting rapid heating comprises two pairs of rolls arranged at a distance in the passing direction of the steel strip, and each pair of rolls holds the steel strip between them and consists of a pair of conductor rolls, or a pressure roll and a conductor roll.
(12) The decarburization annealing facility for a grain-oriented electrical steel sheet having extremely excellent magnetic characteristics, wherein the rapid heating apparatus comprises two pairs of conductor rolls with pinch rolls arranged therebetween, the pinch rolls are provided near the high temperature side conductor rolls, and the steel strip is heated in such a manner that the portion of the steel strip held by the pinch rolls between them has temperatures of up to 750xc2x0 C. and/or a decrease in the temperature of the portion is up to 50xc2x0 C.
(13) The decarburization annealing facility for a grain-oriented electrical steel sheet as disclosed in (9), (10), (11) or (12), wherein nozzles for blowing the atmosphere gas against the steel strip surface are provided in the rapid heating chamber.
The phrase xe2x80x9can oxide film which mainly contains forsterite and is formed on the surfacexe2x80x9d used in the present invention means xe2x80x9can oxide film which mainly contains forsterite and is formed by a reaction with an annealing separator mainly containing MgO and an oxide film which is formed during decarburization annealing at a temperature of more than 800xc2x0 C. with a heating rate of more than 100xc2x0 C./secxe2x80x9d.